1. Field of the Invention
The invention relates to charge and discharge control of a power supply system including a plurality of storage devices.
2. Description of the Related Art
Japanese Patent No. 3655277 discloses a power control system including a plurality of power stages. The power control system includes a plurality of power stages that are connected in parallel with each other and supply DC power to at least one inverter. Each of the power stages includes a battery and a boost/buck DC-DC converter.
In the power control system, the above-indicated plurality of power stages are controlled so as to uniformly or evenly charge and discharge a plurality of batteries included in the respective power stages, so as to achieve a balanced SOC (State Of Charge) for each of the batteries.
However, if the batteries are uniformly charged and discharged, as in the power control system disclosed in Japanese Patent No. 3655277 as identified above, one of the batteries may reach a discharge limit or charge limit at an earlier time than the other battery or batteries in the case where the batteries have different charging and/or discharging characteristics, for example. In this case, the power supply system as a whole is not able to provide the maximum charging characteristic or discharging characteristic after any one of the batteries reaches the charge or discharge limit.
Also, the boost/buck DC-DC converter is provided in each power stage of the power control system as described above, which results in a large increase in the cost. It is, therefore, proposed to use a switching device for permitting some batteries to be selectively connected to one converter, and switch a battery connected to the converter to another one when the SOC of the battery that is currently used is reduced. With this arrangement, however, the battery or batteries in use may be overcharged during regenerative charging unless electric power is appropriately distributed to the batteries in use.